PM synchronous machines are robust, reliable and efficient, and have a relatively high energy density. They have many attractive properties compared with wound-field synchronous machines with brushed or brushless exciters. The need to save energy and increase the efficiency of motors and generators means PM synchronous machines are becoming a popular candidate because of their no-loss rotors. In some cases an energy saving of up to 20% can be achieved by using PM rotors in place of wound rotors.
Despite these advantages, some current designs of PM synchronous machines have shortcomings. They use relatively high volumes of magnetic material that increases their cost. When operated as synchronous generators they have poor voltage regulation and can suffer from distorted voltage and current waveforms under load. When operated as synchronous motors they are difficult to start without some form of sensor feedback. Also, their (stator) back emf waveforms under load are distorted, reducing their efficiency and requiring more complex control electronics.
These problems are greater with low pole number (e.g., 2- and 4-pole) machines, where serious waveform distortion under load commonly occurs. The distortion arises due to the so-called flux drag effect that causes the magnetic flux in the rotor to skew away from the radial axis of the magnets due to the load current in the windings. 2- and 4-pole PM generators also use more magnet material for the same power output than do higher pole number generators. Accordingly, PM synchronous generators have not found widespread use in small-scale power applications, such as single-phase and, three-phase 3000 rpm (2-pole) or 1500 rpm (4-pole) petrol and diesel driven generators in the 5 to 20 kW range.
The only practical current 2- and 4-pole designs are surface-mount magnet types. However, these also have distorted output voltage/back emf and current waveforms under load, and can suffer from demagnetization of the magnet edges under high loads. Surface-mounted magnet segments for 2-pole and 4-pole rotors have a large arc segment, meaning there is a large amount of waste material in their manufacture, increasing the monetary cost of the magnets. Current designs of surface-mount PM AC generators exhibit a large voltage drop under load. As there is no simple method of voltage regulation at constant speed for these machines, generator applications requiring good voltage regulation, cannot use PM synchronous types.
There are, however, PM synchronous generators which have acceptable waveform and voltage regulation. These have one-piece PM rotors. The main problem with these designs is that they use up to six times or more magnet material than the equivalent surface-mount designs, even in the lower powers, and cannot be made in a 4-pole configuration. Rather, such machines are only practical in 2-pole designs up to 3 kW power rating. Above this level the magnet cost and volume increases to the square of the rotor diameter, making them no longer cost-effective.
These problems mean that current designs of PM generators and motors are suitable only for low-grade, low power applications where voltage regulation (i.e., generators) or back emf (i.e., motors), and harmonic distortion are unimportant.
A need exists to overcome or at least ameliorate one or more of these disadvantages.